1. Field of the Invention
This invention relates to a sports equipment, specifically a sports equipment having a rod shape.
2. Description of the Related Art
Recently combined materials of reinforced plastic and metal or the like, which are reinforced with reinforcing fiber such as carbon fiber having characteristics such as excellent resilience, high strength, thermo stability, electrical conductivity and corrosion resistivity, and also low gravity and frictional resistivity, are widely used for aircraft parts, satellite parts, automobile parts, sports equipments or the like. For example, as sports equipments they are used for fancy sports equipments such as fishing sticks, golf shafts, tennis rackets, skis and boats.
An example of especially familiar use of the above reinforced combined material is a golf club shaft of a sports equipment. For golf, a traveling distance and direction stability of the ball are important, and thus easy swing of the golf club is required and reduction in weight of the golf club is aimed. Therefore, as the material used, a reinforced fiber combined material attracts attentions. A golf shaft made of a reinforced fiber combined material can be reduced in weight extensively compared with a golf shaft made of metal (for example steel), moreover having a greater degree of freedom of design, for example setting twisting rigidity and bending rigidity relatively freely, and allowing to design a degree of bending according to a golf player's level.                [Patent Document 1] Japanese Laid-Open Patent Publication No. 2004-130564        
However, a golf club shaft comprising prepreg layered in a conventional method disclosed the patent document 1 is improved in reduction in weight as well as impact strength and bending strength, but there is a certain limit. Therefore, a golf club shaft made of a reinforced fiber combined material is preferably used for females who prefer a light-weighted golf club or golf players who do not have much strength, however a golf club shaft reduced in weight by decreasing a number of layers of prepreg does not have strength, so that bending occurs more than necessarily, and the swinging track does not remain stable. As a result, tracks of the head and the face become unstable (for example, so called a Toe Down phenomenon, that the head end goes down because of the weight of the head itself or the like), and thus it is very difficult to hit a ball to a desired direction because a degree of the face is messed up. Whereas, powerful golf players, such as experienced players or professional golfers, seek further strength of the golf club shaft, and their fast swing of the club deflects the golf club shaft too far, and further its poor resilience makes the head move slightly slower than the player's intention. Also, deflection and vibration of the golf club shaft occur by the impact when it hits a ball. Therefore, because of the opinions that the ball direction is not stable and it's difficult to handle, it is a current situation that these golf players still use golf club shafts made of metal.
FIG. 15 is a conceptual drawing showing a inner structure of a golf club shaft 100, and FIGS. 16 A to D are drawings showing a example of manufacturing processes of the golf club shaft 100. As shown in FIG. 15, the golf club shaft 100 made of a reinforced fiber combined material is ideally formed by layering repeatedly bias layers 102, having a fiber direction of reinforced fiber 101 thereof being inclined at a predetermined degree to an axis direction O, and straight layers 103, having a fiber direction of the reinforced fiber 101 thereof along the axis direction O.
By the way, the above mentioned bias layers 102 and the straight layers 103 are, as shown in FIGS. 16 A to D, formed by sequentially layering sheet-formed reinforced fiber combined materials cut into a predetermined shape rolled around a core rod 104, for example being made of metal with a tapered shape having a larger diameter to a axis direction, so as to be layered. Most developers believe that this golf club shaft 100 is formed having the above mentioned fiber direction, actually the fiber directions of the reinforced fiber of the sheet-shaped reinforced fiber combined materials are actually not at all as mentioned above, and no one notices this fact. This produces serious problems that the shaft deflects too much as well as that the resilience to the deflection is weak.
Specifically, in a case of rolling the straight layer, as shown in FIGS. 16 A and B, it starts to be rolled, after disposing the reinforced fiber combined material having the fiber direction following the axis direction of the core rod 104. Next, continuing rolling the reinforced fiber combined material around the core rod 104 results, as shown in FIG. 16 C, because of the core rod 104 having a tapered shape in the axis direction, that the reinforced fiber 101 disposed along the axis direction and lined substantially parallel is not straight along the axis direction on a circumferential surface of the tapered shape, and the fiber direction slopes within one round of the rolling-up, so that, as shown in FIG. 16D, the reinforced fiber 101 overlaps in a spiral manner (bias layer), resulting in that it is rolled up merely irregularly. Accordingly, to the axis direction which requires strength most, there is no straight layer 103 exists, as shown FIG. 15, which is supposed to provide bending (deflection) rigidity of the shaft, so as to not be able to secure rigidity, and there is a certain limit of improving the impact strength and the bending strength.
Also, it is still possible to form the above mentioned golf club shaft having the fiber direction of the reinforced fiber overlapping irregularly by overlapping this reinforced fiber combined material repeatedly in order to increase strength, so as to have the same degree of the strength as a metal shaft. However, overlapping it repeatedly makes the entire golf club shaft thick and hard, and resulting in deterioration of its resilience and further increase in weight of the shaft itself, so that there is a dilemma of losing the most advantage of the reinforced fiber combined material, which is reduction in weight of the golf club shaft.
The problem of this invention is to provide a sports equipment made of a reinforced fiber combined material, specifically used for a golf club shaft, maintaining a characteristic of light-weight as well as securing the same or higher strength as a golf club shaft made of metal, and rapid resilience to deflection, such as golf club shaft provided with high strength such as bending rigidity and twisting rigidity capable of suppressing unnecessary bending and vibration.